The physiological disposition and metabolism of tetrahydrouridine (NSC-112,907) (THU) will be studied with and without ara-C in experimental tumor systems and in patients with the aim to improve drug schedule, tissue sensitivity and tissue specificity of 1-beta-D-Arabinofuranosylcytosine (ara-C). The approaches to be used will be the following: 1. In vivo chemotherapeutic evaluation of ara-C and THU and distribution of C14-THU alone or in combination with H3-ara-C in experimental tumor systems with deoxycytidine (dCR)/ara-C-kinase and cytidine (CR)/ara-C deaminase activity comparable to human leukemias. 2. In vivo study of the physiological distribution of C14-THU in humans and C14-THU in combination with H3-ara-C.